linux-kernel - Re: [PATCH] x86/uaccess: Use pointer masking to limit uaccess speculation

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Message-ID: <20200901145203.GB95447@C02TD0UTHF1T.local>
Date:   Tue, 1 Sep 2020 15:52:03 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Will Deacon <will@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] x86/uaccess: Use pointer masking to limit uaccess
 speculation

On Tue, Sep 01, 2020 at 09:21:58AM -0500, Josh Poimboeuf wrote:
> On Tue, Sep 01, 2020 at 03:02:08PM +0100, Mark Rutland wrote:
> > d instead do user pointer
> > > masking, throughout the x86 uaccess code.  This is similar to what arm64
> > > is already doing.
> > > 
> > > barrier_nospec() is now unused, and can be removed.
> > 
> > One thing to consider is whether you need a speculation barrier after
> > set_fs(). Otherwise for code like:
> > 
> > | fs = get_fs();
> > | if (cond)
> > |	set_fs(KERNEL_DS);
> > | copy_to_user(...)
> > | set_fs(fs)
> > 
> > ... the set_fs() can occur speculatively, and may be able to satisfy
> > the masking logic if forwarded within the cpu.
> > 
> > See arm64 commit:
> > 
> >   c2f0ad4fc089cff8 ("arm64: uaccess: Prevent speculative use of the current addr_limit")
> 
> Do you have any examples of that conditional set_fs(KERNEL_DS) pattern?
> I wasn't able to find any.

I'm afraid not -- we used to in arm64 in some memory dump code, but that
is now gone. It might be that this is no longer necessary.

If set_fs() goes entirely, that's even better...

Thanks,
Mark.